Functional polymers are promising materials for high-technology innovations, and developing new synthetic routes for these polymers is an important research area in macromolecular science. Incorporation of sulfur into the structure of macromolecules is one method to obtain advanced functional materials, since sulfur-containing macromolecules bear advanced properties. However, incorporation of sulfur moieties into a macromolecular structure may also encounter many problems like reactivity, solubility, difficulty in synthesis of monomers, and complicated steps in obtaining the target macromolecules.
Hydrothiolation of an alkyne is a reaction where thiol and acetylene react with each other and form a vinyl product. Vinyl molecules are of interest to many scientists due to their novel properties, such as their insulating properties. In 1956, Truce and Simms discovered a nucleophilic reaction between aromatic or alkyl acetylene with sodium thiolate, which gives vinyl sulfides in high yields (William E. Truce and Joh A. Simms, J. Am. Chem. Soc. 1956, 12, 2756-2759). This reaction was later termed “Alkyne Hydrothiolation”. In 1987, Oshima, Ichinose and coworkers successfully utilized triethylboron as a radical to initiate the reaction of acetylenes and thiols (Ichinose et al., “Et3B Induced Radical Addition of Thiols to Acetylenes,” Chem. Lett., 1987, pp. 1647-1650). Transition-metal complexes were employed for such reactions, but the early attempts were met with little success because of the poisonous interactions of sulfur atoms with metal catalysts.
Newton succeeded in adding thiophenol to a highly reactive alkyne molecule using molybdenum as a catalyst (McDonald et al., “Catalysis by Molybdenum Complexes. The Reaction of Diazenes and Acetylenes with Thiophenol,” Inorg. Chem., 1976, Vol. 15, No. 9, pp. 2056-2061). However, using this reaction the product was obtained in a relatively low yield. In 1992, Ogawa and Sonoda (Akiya Ogawa and Noboru Sonoda et al., J. Am. Chem. Soc. 1992, 114, 5902-5903) revealed that rhodium and palladium complexes can effectively catalyze such a reaction, producing both branched and linear vinyl sulfides as products. Since then, alkyne hydrothiolation has become one of the most widely studied reactions and many radicals, nucleophiles, and metal complexes have been found to work as catalysts by different mechanisms (Jan-E, Backvall and Anna Ericsson, J. Org. Chem. 1994, 59, 5850-5851; Li-Biao Han et al., J. Am. Chem. Soc. 2004, 26, 5080-5081; Akiya Ogawa et al., J. Am. Chem. Soc. 1999, 121, 5108-5114; Suzanne Burling et al., Dalton Trans. 2003, 4181-4191).
Macromolecules with a high refractive index are potential candidates for applications in photo-optical devices. Target macromolecules need to have good film forming ability, high transparency, a high refractive index and high thermal stability. One method to obtain high refractive index macromolecules is to incorporate sulfur or other hetero atoms, like nitrogen, oxygen, etc., into the structure of macromolecules.
Different research groups have synthesized sulfur-containing macromolecules by various means, by simply mixing different ingredients (WO 2007/088556 or by incorporating sulfur into a monomer structure (Rie Okutsu et al., Macromolecules, 2008, 41, 6165-6168; Nam-ho You et al., Polymer, 2009, 50, 789-795). Another method to obtain high refractive index macromolecules is to incorporate metal moieties into the structure. With the addition of metal, the refractive index of the macromolecules can be greatly enhanced (US 2009/0111677). However, this usually results in the low processability and the low transparency of the macromolecule to be targeted.
Hydrothiolation of acetylene with thiol may produce branched and linear vinyl sulfides in high yields. Scientists have actively explored catalysts for this reaction to synthesize vinyl sulfides with different substituents; however, little effort has been developed toward its utilization in the synthesis of new macromolecules. In addition, alkyne hydrothiolation of acetylenes and thiols with triple and mercapto bonds, equal to or greater than two, may generate polymers with linear and hyperbranched structures. By using a stepwise reaction, dendrimers containing such functionality can also be obtained. Such possibilities, however, have not yet been explored, despite the fact that such macromolecules may exhibit novel material properties. Accordingly, there have been needs for a new method for preparing sulfur-containing macromolecules having new properties by employing alkyne hydrothiolation.